1. Field of the Invention
The present invention relates to an evoked respons detector for a heart stimulator for determining evoked response In the presence of polarization, the heart stimulator being of the type having a pulse generator for producing stimulation pulses of varying amplitudes and varying durations for stimulating the heart of a patient, and the evoked response detector having measuring and memory means for measuring the charge delivered by a stimulation pulse, The invention also relates to a heart stimulator having such an evoked response detector.
2. Description of the Prior Art
There is a need to make unipolar pacemakers, both for ventricular and atrial stimulation of the heart of a patient, that have a so-called AUTOCAPTURE.TM. pacing system function, The AUTOCAPTURE.TM. function is used to maintain the energy of the stimulation pulse at a level just above the level which is needed to effectuate capture, described, for example, in U.S. Pat. No. 5,458,623. In this context it is difficult to detect evoked response in a safe and reliable manner, since the evoked response potential is small in amplitude compared to the residual polarization after the stimulation pulse and this polarization is varying when the stimulation energy is varied for the threshold search. The polarization is also varying with variations of the impedance.
Several attempts have been made to solve the polarization problems in connection with evoked response detection. Thus U.S. Pat. No. 5,417,713 discloses a system for maintaining capture, wherein an electrical post-stimulus signal of the heart following delivery of a stimulation pulse is compared to a polarization template, determined during a capture verification test. A prescribed difference between the polarization template and the post-stimulus signal then indicates capture. Otherwise loss of capture is presumed and the stimulation energy is increased a predetermined amount to obtain capture.
One technique of reducing the effects of polarization is to remove the polarization charge by supplying after the delivery of the stimulation pulse one or more suitable pulses of opposite polarity, see U.S. Pat. No. 4,811,738.
U.S. Pat. No, 5,431,693 describes a method for verifying capture by a pacemaker. This method is based on the observation that the non-capture potential is exponential in form and the evoked capture potential, while generally exponential in form, has one or more small-amplitude perturbations superimposed on the exponential wave form. These perturbations are enhanced for facilitating detection by processing the waveform signal by differentiation to form the second derivative of the evoked response signal for analysis for the evoked response detection.
Unipolar detection of evoked response signals is, however, not possible by this technique. Abrupt slope changes or superimposed small-amplitude perturbations are leveled out when the measurements are made over a longer distance from the electrode to the stimulator casing. In Swedish patent application No. 9703600-8 (corresponding to pending U.S. application Ser. No. 09/161,665 filed Sep. 29, 1998 ("Heart Stimulator With An Evoked Response Detector," Budgifvars et al.,) and European Application 98116180.5) an evoked response detector is described which functions based on the fact that the evoked response signal amplitude does not vary significantly with the amplitude of the stimulation pulse (provided that the stimulation amplitude is above the capture threshold) whereas the electrode polarization is approximately linearly dependent on the stimulation pulse amplitude for a constant pulse duration within a certain stimulation pulse amplitude range. This constancy versus linear dependency dichotomy is used to distinguish the evoked response signal from the polarization signal.